A sonic boom from a plane? A stick of dynamite? Somebody blowing up old tree stumps? Something at the refinery? Blasting for a new logging road? Mount Baker getting ready to erupt? How about a UFO?

Plenty of theories were floated on social media Monday night and Tuesday morning about the cause of a large boom that was heard from Everson to Bellingham and west to Blaine around 7 p.m. Monday. According to reports, the boom shook houses, though no reports of damage were mentioned.

As of Tuesday afternoon, the only thing known for sure was that it was loud. The cause is still a mystery.

"We are looking into it and tracking down leads as to the exact location(s) where they may have occurred," Whatcom County Undersheriff Jeff Parks said in an email Tuesday.

Whatcom County Fire District 1 Chief Mel Blankers said the blast was felt at the firehouse in Everson and that they saw some smoke afterward.

"That was a permitted burn where they were burning some flash piles," Blankers said. "It did not appear to be related to the explosion."

Blankers said the Whatcom County Fire Marshal was still investigating the source of the boom.

Does anyone have any info on the large explosion heard and felt in Whatcom county?